The present invention relates to a magnetoresistive sensor. More particularly, the present invention relates to a magnetoresistive sensor including a layer system having a measuring layer, a bias layer, and an interlayer, and including measuring contacts on the layer system.
In ferromagnetic transition metals such as nickel (Ni), iron (Fe) or cobalt (Co), and in alloys containing these metals, the electrical resistance depends on the magnitude and direction of a magnetic field permeating the material. This effect is referred to as anisotropic magnetoresistance (AMR) or anisotropic magnetoresistive effect. This is physically based on the different scattering cross sections of electrons having different spins which correspondingly are referred to as majority electrons and minority electrons of the D band. A thin layer made of such a magnetoresistive material having a magnetization in the plane of the layer is generally used for magnetoresistive sensors. The change in resistance as the magnetization rotates with respect to the direction of the current may amount to several percent of the normal isotropic resistance.
Multilayer systems are known which comprise a plurality of ferromagnetic layers which are arranged in a stack and are separated from one another by metallic interlayers, and whose magnetizations in each case coincide with the plane of the layer. The respective layer thicknesses in this arrangement are chosen to be considerably smaller than the mean free path of the conduction electrons. In such layer systems there then arises, in the individual layers, in addition to the anisotropic magnetoresistive effect, the so-called giant magnetoresistive effect or giant magnetoresistance (Giant MR), which is due to the differential scattering of majority and minority conduction electrons in the bulk of the layers, especially in alloys, and at the interfaces between the ferromagnetic layers and the interlayers. This Giant MR is an isotropic effect and may be considerably larger than the anisotropic MR, with values of up to 70% of the normal isotropic resistance.
Two basic types of such Giant-MR multilayer systems are known. In the first type, the ferromagnetic layers are antiferromagnetically coupled to one another via the interlayers, so that those magnetizations of two adjacent ferromagnetic layers, which coincide with the planes of the layers, align themselves antiparallel with respect to one another in the absence of an external magnetic field. An example of this type of Giant-MR multilayer systems are iron-chromium superlattices (Fe--Cr superlattices) having ferromagnetic layers consisting of Fe and antiferromagnetic interlayers consisting of Cr. An external magnetic field causes the magnetizations of adjacent ferromagnetic layers to rotate against the antiferromagnetic coupling forces and to align themselves in parallel. This reorientation of the magnetizations by the magnetic field results in a steady decrease of the Giant MR, which decrease is a measure for the magnitude of the magnetic field. Once a saturation field strength H.sub.S is reached, no further change in the Giant MR takes place, because all magnetizations are then aligned in parallel with respect to one another. The Giant MR in this situation depends solely on the magnitude of the field strength ("Physical Review Letters", Vol. 61, No. 21, Nov. 21st 1988, pages 2472-2475).
This type of Giant-MR multilayer system comprising antiferromagnetically coupled ferromagnetic layers has also been the subject of theoretical calculations which show that the current coefficients and the transmission coefficients for spin-up electrons scattered at the interfaces and similar spin-down electrons depend on the angle between the magnetizations in adjacent ferromagnetic layers. According to these calculations, the Giant MR increases steadily as the angle between the two magnetizations increases from 0.degree. to 180.degree., and is greatest at an angle of 180.degree. ("Physical Review Letters", Vol. 63, No. 6, August 1989, pages 664-667).
In the second type of Giant-MR multilayer systems, ferromagnetic layers whose magnetizations in the planes of the layers are, on average, parallel to one another, are separated from one another by diamagnetic or paramagnetic interlayers consisting of metal. The thickness of the interlayers is chosen to be sufficiently large for the magnetic exchange coupling between the magnetizations of the ferromagnetic layers to be as small as possible. In each case, adjacent ferromagnetic layers have different coercive field strengths. As a result, the mean values, which are initially parallel in saturation, of the magnetizations M.sub.1 and M.sub.2 of magnetically softer measuring layers and adjacent, magnetically harder bias layers are rotated differentially by a magnetic field H, and an angle Phi between the mean values of the magnetizations M.sub.1 and M.sub.2 is established which depends on the magnetic field H. The dependence of the individual magnetizations M.sub.1 and M.sub.2 on the magnetic field H in the process results from the corresponding hysteresis curves of the magnetically softer and the magnetically harder material. Between the coercive field strengths H.sub.c1 of the magnetically softer and H.sub.c2 of the magnetically harder layers, and between --H.sub.c2 and --H.sub.c1 there is in each case a region in which the magnetization Mm is already saturated and the magnetization M.sub.2 still has its value corresponding to saturation and is aligned antiparallel to the magnetization M.sub.1, i.e., Phi=180.degree. In this region the MR signal is at a maximum and constant. Different coercive field strengths .vertline.H.sub.c1 .vertline..ltoreq.H.sub.c2 .vertline. can be set by selecting different materials or by different fabrication processes or the selection of different thicknesses of the same material. Known layer structures comprising different materials are, for example, NiFe--Cu--Co layer structures and Fe--Cu--Co structures. A known layer system based on different fabrication or different thicknesses is a Co--Au--Co system ("Journal of Applied Physics", Vol. 70, No. 10, Nov. 15, 1991, pages 5864-5866). The MR signal of these known layer systems then, however, depends on its previous history, i.e., along which path and between which values for the magnetic field and in which direction the hysteresis curves are traversed. On the basis of such a layer system it is therefore not possible to implement an MR sensor having an unambiguous, reversible characteristic curve. Moreover, in these known layer systems part of the magnetic flux of the harder bias layers forms a circuit above the softer measuring layers. This magnetic interference field reduces the measuring sensitivity of the sensor and results in an undesirable shift of the sensor characteristic curve.
European Patent Document No. EP-A-0 346 817 discloses a magnetoresistive sensor including a layer system comprising a ferromagnetic measuring layer and a ferromagnetic bias layer which, by a non-magnetic interlayer, are exchange-decoupled from one another. The magnetoresistive sensor also includes measuring contacts at the layer system for applying an electric current and taking off a measured voltage which is a measure for an applied magnetic field. In a first embodiment of this known magnetoresistive sensor the measuring layer has a smaller coercive field strength than the bias layer. In a second embodiment, in contrast, an antiferromagnetic layer lies against the bias layer. The measuring layer therefore has, in the plane of the layer, a rotatable magnetization which, within the measuring range of the magnetic field, depends inversely on the magnetic field, whereas the bias layer in its plane has a magnetization which is constant in the measuring range.